In existing reinforced concrete elements or members such as concrete slabs, beams, columns and walls, it is sometimes desirable to strengthen the member for one or more reasons. For example, the applied loading requirements may exceed the original design values for the member, or the load carrying capacity of the member may have been reduced due to deterioration, or the member may require increased stiffness for less deflection. The member may also require lower working stresses to reduce fatigue, or may require upgrading to withstand higher seismic and/or blast loading.
One form of strengthening existing reinforced concrete elements or members is by laminating or bonding a mat or strip of composite material with carbon or glass fibers to the surface of the concrete member where bending occurs. However, it is undesirable for the composite mat or strip to be exposed to the weather and/or to traffic such as on the top surface of a concrete bridge slab. For example, if water seeps between the composite mat or strip and the concrete surface, it is possible for the mat or strip to delaminate from the concrete surface if the water freezes. It is also necessary to prepare the concrete surface in order to obtain a good bond of the reinforcing mat or strip to the concrete surface.
Another form of strengthening a reinforced concrete horizontal deck is by cutting or forming parallel grooves or cavities within the surface of the concrete deck and embedding steel reinforcing rods or xe2x80x9crebarsxe2x80x9d within the grooves or cavities with mortar or concrete, for example, as disclosed in U.S. Pat. No. 4,574,545 and as described in an article entitled xe2x80x9cStrengthening Bridge Slabs with Grouted Reinforcementxe2x80x9d published in the January 1949 issue of the xe2x80x9cJournal of The American Concrete Institutexe2x80x9d. It is also known to reinforce a stone slab with a series of cables embedded in parallel spaced grooves formed or cut in a back surface of the stone slab, with the cables being embedded in a mixture of sand and epoxy resin which filled the grooves, as disclosed in French Patent No. 2 562 927. Wood beams and wood planks have also been reinforced with a polyester rod or rods which are glued within a groove or grooves formed within a surface of the wood beam or plank, for example, as disclosed in U.S. Pat. Nos. 5,565,257 and No. 4,615,163.
The present invention is directed to an improved method for strengthening existing reinforced concrete elements or members such as concrete slabs, beams, columns and walls after it is determined where the existing tensile reinforcing rods or bars in the concrete are inadequate. In accordance with the invention, one or more parallel spaced shallow grooves are cut within the surface of the existing reinforced concrete element or member in the direction of bending of the member and in the area of inadequate tensile reinforcing. A composite fiber reinforcing element, such as a composite rod or element with continuous carbon fibers, is positioned within each groove after a polymer adhesive resin or epoxy resin is inserted into the groove. The reinforcing rod is twisted or rotated or shifted so that the adhesive resin completely surrounds the reinforcing element. The polymer adhesive resin is formed flush with the surface of the concrete member and allowed to cure to bond each element or rod to the concrete defining the corresponding groove. Each groove and corresponding reinforcing element or rod extend within the top surface of a concrete slab across a support for the slab and extend within the bottom surface of the slab at least fifty percent of the distance between adjacent supports for the slab. Each groove and reinforcing element may also extend within a vertical surface of a masonry or concrete wall in the direction of bending of the wall.
The method of the invention eliminates surface preparation of an existing concrete member, a step that is normally required to bond a strip or mat to the element or to connect a steel rebar within a concrete groove. The method also provides for locating the supplemental reinforcing element or rod just below the concrete surface, thereby protecting the reinforcing element or rod which is completely encased within the polymer adhesive or epoxy resin. The fiber reinforcing element is also located adjacent or just under the surface of the concrete where the element is the most effective. Also, the resin within the grooves is protected from heat due to fire and is less likely to soften and lose strength, in comparison to the exposure of the resin which is used to attach strips or plates to the concrete surface.
The invention further provides for concentrating the reinforcing elements at the critical stress locations, and the use of a composite element with continuous fibers for the supplemental reinforcing provides for efficient use of the supplemental reinforcing adjacent the surface of the concrete element. The supplemental reinforcing rods within the grooves may also be pre-stressed before adhesively bonding to the concrete, and the concrete element may be deflected in a direction opposite to the direction of deflection caused by loading of the concrete element to provide for an initial pre-stressing of the reinforcing rod or element.
Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.